The Science of Why Beer Is So Delicious

Whether you're catching a whiff of banana from a tall glass of Hefeweizen or enjoying the subtle floral aromas in your favorite American IPA, you have yeast to thank for it. Yeast imbues beer with aromatic molecules that account for a great deal of its final flavor; almost all wild yeasts create these pleasant smells and tastes as they eat.

Yet even though we know yeast is the reason beer tastes so good, we don't know exactly why it does it. But in a new study, a team of scientists led by Kevin Verstrepen, a yeast geneticist at the Flanders Institute for Biotechnology and the Belgian University of Leuven, has showed why these tiny microbes make the flavors we savor.

In a new paper in the journal Cell, the scientists detail the results of four experiments on yeast. It turns out that for yeast, producing these delicious aromatic molecules is a bit like hailing a taxi. The smell lures in wandering flies, to which yeasts hitch a ride so they can disperse throughout nature.

Serendipity and Dirty Lab Equipment

"This theory actually came about entirely by accident," Verstrepen says. Fifteen years ago, working on his Ph.D., he was investigating the gene that controls the production of these aromatic molecules in yeast. "At one point on a Friday night, I was in hurry to get to the bar and didn't clean up my experiment as well as I should have," he says.

He accidentally left out three flasks of yeast. "One flask had a mutant strain of yeast that produced 100 times more [of these aromatic molecules], one flask contained normal yeast, and a third had a yeast with this aroma gene partially knocked out," Verstrepen says.

"When I returned Monday morning, I found that somehow fruit flies had gotten into the lab," he says. And realizing that he had unwittingly set up an experiment, he wrote down the results. "Fifty flies drowned in the mutant flask, two drowned in the flask of normal yeast, and the flask with the [nonaromatic] yeast didn't have any," he says. "You don't have to be a genius to start to draw some conclusions from that."

All these years later Verstrepen teamed up with a group of fly neurobiologists to test his idea. They did it in four parts.

"The Arena"

1. In the first of Verstrepen's experiments, the scientists identified and picked out a particularly potent aroma-producing yeast strain. They then knocked out the gene called ATF1, which is responsible for creating the bulk of the yeast's aromas, rendering it more or less scentless. The scientists then grew a culture of the scentless yeast as well as a culture of the unmodified strain, and placed a tray of each in opposite corners of an airflow-regulated box they had built.

The scientists call it the arena. I am not kidding.

When the researchers then introduced 50 gladiators—er, fruit flies—into the arena and recorded their flights and landings with cameras, they found—just as Verstrepen found 15 years ago—that the flies overwhelmingly preferred the fruity, aromatic yeast.

2. In the second experiment the scientists wanted to make sure it was the aromatic molecules—and not some other unidentified enticement—luring the flies. So Verstrepen and colleagues set up the arena as before, but this time they added the aromatic molecules back into the scentless yeast tray—so that the aromas of both trays would be identical. Indeed, when the fruit flies entered the arena, they showed no preference.

3. For the third test the team peered inside the brain of the flies as they smelled various aromatic, yeast-produced molecules. "It's actually a really cool setup," Verstrepen says. The researchers used flies with proteins in their brains that phosphoresce (think, glow in the dark) with neural activity. "So you can slide open the skull of the flies [and] monitor their brain activity by literally watching their brains flash up phosphorescently," Verstrepen says. Using this technique, the researchers discovered that one chemical—isoamyl acetate—made the flies' brains go wild. And this should surprise absolutely no one who's ever had fruit flies in his kitchen. Isoamyl acetate smells like overripe bananas.

4. Verstrepen had thought that the evolutionary advantage yeast gains from luring flies is that the flies help the yeast spread through its environment. To test this in the final experiment, the scientists placed a tray of both the normal and scentless yeasts (colored with different dyes for easy recondition) at opposite sides of an enclosed cylinder, introduced flies, and left it alone for 48 hours. After the two days the scientists took a snapshot of the smears and found that the aromatic yeast had smeared across four times as much area as the scentless yeast. The aromas made a huge difference in mobility.

"It's certainly a very intuitive mechanism," says William Alexander, a yeast researcher at the University of Wisconsin who was not involved in the paper. Alexander explains that for yeast, which lacks any cellular components like flagella to help it get around, the evolutionary benefit of being able to spread quickly through insects "to fruit just as it becomes ripe or a tree when it starts leaking sap, is enormous."

Making Beer From Crushed Flies

The best and strangest part of Verstrepen's research is something that didn't even make the paper.

Out of sheer curiosity, Verstrepen gave his colleagues fly traps and asked them to collect samples around their houses and apartments. When they returned with now-dead wild flies, Verstrepen ground up the flies and analyzed the resulting insect mash for yeasts. Sure enough, he found some. He cultured these yeasts and found them to be pleasantly aromatic.

Then he did what any sane person would do, having pulled yeasts from crushed up insects. "We made beer with them," Verstrepen says.

Apparently it was delicious. I'd still argue that Verstrepen now rivals the guy who made beer from his beard yeast in the contest for the world's strangest brewer. But I asked William Alexander (our outside yeast expert) about this story, and he was unimpressed with my squeamishness.

"To grab flies and throw them in your [brew]—that's probably much more reminiscent of how humanity made beer for the last 10,000 years than any of the industrial technologies we now use today," he says.

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